1. Field of the Invention
This invention relates to cardiac pacemakers and, more particularly, to implantable cardiac pacemakers with automatic means for discriminating between intermittent noise and true sensed cardiac signals.
2. Description of the prior Art
For virtually the entire lifetime of the cardiac pacemaker industry, there has been a need to make pacemakers more invulnerable to sensing externally generated noise. With the advent of the demand pacemaker, pacemakers have included a sensing circuit for sensing natural heartbeats, so as to inhibit delivery of pace pulses in the presence of naturally occurring beats. But such a sense circuit inherently is able to pick up other signals as well, and it has thus been necessary to design protection into the pacemaker so that externally generated noise of any nature is not interpreted as heartbeats.
There have been many approaches to the problem of discriminating noise from true cardiac signals. The filter characteristics of the sense amplifier can be adapted to maximize detection of the cardiac signal--either the QRS or P wave--while filtering out signals that have characteristics more similar to noise. See also U.S. Pat. No. 4,379,459, which discloses a pacemaker with detection circuitry that senses intrinsic heart signals in the presence of very large amplitude repetitive noise signals. Another long used approach is disclosed in U.S. Pat. No. 4,043,347. This reference illustrates the standard noise suppression technique of resetting a flipflop, or other logic circuitry, upon the detection of noise, so as to extend the pacemaker refractory interval and thereby prevent the pacemaker from acting on early signals which could be noise. In such an arrangement the detection of continuous noise prevents any signal from the sense amplifier from resetting the refractory interval, such that substantially continuous noise results in fixed rate pacing. However, this arrangement cannot accurately respond to discontinuous, or intermittent noise of many varieties, e.g., noise spikes that are detected after the refractory interval.
The above problems take on more urgency in view of the Cenelec standard for safe pacing during intermittent noise. What is needed is a more reliable noise detection arrangement for handling intermittent as well as continuous forms of noise, and determining when a sense signal can be treated as a true intrinsic cardiac signal and when it must be treated as noise.